D3728: grep: adds allfiles mode

[Yuya Nishihara]

> +    if allfiles and len(revs) > 1:
> +        raise error.Abort(_("multiple revisions not supported with --allfiles"))
>      wanted = set()
>      slowpath = match.anypats() or (not match.always() and opts.get('removed'))
>      fncache = {}
> @@ -1990,7 +1994,11 @@
>                  ctx = change(rev)
>                  if not fns:
>                      def fns_generator():
> -                        for f in ctx.files():
> +                        if allfiles and len(revs) == 1:

Removed `len(revs) == 1` here and queued, thanks.